Plugins attribute should not be required if test container points to a plugin project

[kapexx]

Related to #79, if a junit-plugin container points to a plugin project, it is added to the list of dependencies. If there are no explicit plugin dependencies defined though, we get the error missing required attribute: [plugins, contentProviderProduct, features]. This seems to restrictive to me, it should be ok to omit explicit dependencies in that case.

For example, this should be a valid configuration, but it is currently considered invalid because org.example.demo.test is only listed as container but not redundantly as plugin dependency.

junit-plugin configuration Demo {
    test {
        runner junit5;
        container '/org.example.demo.test';
    }
    application org.eclipse.pde.junit.runtime.coretestapplication;
}